How will you have to carry it?
If you need to carry it around in a small space, such as a changing bag, a fabric seat harness would work well. It’s very compact once folded and won't add much to the weight of what you’re already carrying.
If you have a buggy basket you can store it in then you have a bit more choice and could find either have a foldable booster seat, small booster seat, flat-folding clamp-on seat, flat-folding travel highchair or a seat harness are fine.
If you’re usually driving to wherever you’re eating then it doesn't really matter what you choose. However, if you’re packing for a holiday, remember to take into account all the other baby paraphernalia that will have to go into the car boot, too!
Before you decide, check the weight, folded size and whether it comes with a carry handle or a bag you can easily transport it in. Even if a product is light, it won’t be easy to carry if it’s a cumbersome size – especially if there’s also a buggy, toddler and changing bag to juggle!